1. Field of the Invention
With respect to the classification of art as found in the United States Patent and Trademark Office the present invention pertains to the general Class entitled, "Turning" (Class 82) and more particularly to the subclass therein entitled, "centers" (subclass 33).
2. Description of the Prior Art
Live centers are well known and generally fall within three categories, light, medium and heavy-duty depending upon the capacity. These classifications will hereinafter be more fully described. In general the live centers as shown in the art and as used by industry accommodate up to 10 tons very satisfactorily. However, when the capacity exceeds 10 tons and up to 250 tons such heavy loads shorten the life of the bearing in the live centers particularly when radial bearings are employed. Centers requiring extremely heavy-duty load capacity are generally highly stressed and where the rotating spindle of the live center accepts and supports a very heavy work load the concentrated load upon these bearings becomes extremely high and their life and accuracy are shortened. It is a purpose of this invention to provide, and it does provide, a heavy-duty live center which will accept and continue to operate with ultra precision while supporting a very heavy workpiece load of from 10 to 250 tons.
In order to establish definition for the purpose of this description, a live center is a work holding device having a center point carried in the spindle mounted in antifriction bearings with the center point supported and rotating with the workpiece.
Conventionally there are three categories of live centers and these three categories of live centers are illustrated in the drawings. The light-duty live center is adapted to be used with a 1,000 pound or less workpiece and generally has two bearings mounted back to back. The medium-duty live center will accept up to 10 tons in a workpiece and includes several bearing designs, usually one or two front bearings and one or more rear bearings with the front bearings often being roller-type bearings. Finally the heavy-duty live center accepts from 50 to 250 ton workpieces. These heavy-duty live centers have quill-type bodies with very heavy-duty bearing construction. Where the required carrying capacity is approximately 35 tons, the smallest in the heavy-duty category, at least a 36 jarno taper which is about 41/2 inches at the gauge line is used. For this heavy-duty capacity and up to 250 tons there are generally three bearings at the foreportion of the live center and the bearings closest to the center point are called the front bearings and those toward the shank are called rear bearings. Heavy-duty live center designs usually incorporate needle or straight roller front bearings which have a very heavy radial capacity. When such bearings are used periodic adjustment of the bearings according to preload is necessary to compensate for bearing wear. It has been found that when a disassembly of the live center is required for bearing adjustment, which disassembly is seldom performed, it is often unsatisfactory. After the adjustment has been performed the center point will no longer maintain its true center but must be reground. Tool pressure and other loads cause the center points of improperly adjusted bearings in quills to become eccentric. Deep group ball bearings may have a good radial capacity but are not suitable for front bearings since a radial adjustment cannot be made. As the bearing wears the center point starts to move erractically causing an outrun of the workpiece. Where two tapered roller bearings are used, with the rear bearing mounted to take the thrust, looseness also develops from wear. This allows the spindle to follow a planetary motion unless bearing preload is carefully adjusted as wear takes place. A front tapered roller bearing assembly is best suited for absorbing both radial and axial loads due to the wedging of the bearings under thrust. A live center employing this type of front bearing is especially suited for cylindrical grinding where neither thrust nor rotating speed is too high. Tapered roller bearings are less suitable for lathe work due to their inherent poor thrust capacity and heat buildup at higher turning speeds. It is also necessary to have automatic preload on a bearing of this type to sustain accuracy and achieve maximum bearing life.
Nearly all live center manufacturers provide bearing preload at assembly. Bearing adjustment due to wear is handled in two ways, manual and automatic. When manual adjustment is internal the live center must be disassembled. This is impractical. Even when the preload arrangement is accessible adjustment for wear is nearly always neglected. Centers with an automatic preload arrangement require neither disassembly nor manual adjustment. Such a center is provided in the present application.
For heavy work loads the spindle should be supported on approximately a 1 to 4 ratio. A self-aligning construction is essential to safeguard against damage to the bearings since the spindle will bow under a heavy work load.
For medium work loads a short heavy spindle works best. This type of spindle eliminates excessive bowing and deflection which is damaging to bearings. Some live center designs incorporate a long slender spindle. This spindle may flex under heavy load and cause rapid deterioration of some or all of the bearings. When the spindle flexes thrust is no longer distributed evenly to the thrust carrying bearings since only a few balls now support the work load and a force is created by the flexing of the spindle. Premature bearing failure results. Excessive flex has been found to cause the spindle to fatigue to the point of breakage.
Normally a live center's capacity rating is based on the total workpiece weight. This is often misunderstood since a workpiece is supported on both ends and the live center is designed to support only half of the total workpiece weight. Tool pressure and thrust are also to be considered. To support a workpiece between 60.degree. center points requires a minimum end thrust of 25 percent of the workpiece weight. Since end thrust increases from heat expansion it is recommended that a center with a high thrust capacity be used. When a bearing is subjected to a load of only one-half its maximum rating the life expectancy of that bearing increases 8 times. Using a live center with more capacity than needed is a safeguard against bearing failure due to overload.
Venting of the internal cavity of a live center is necessary to prevent the exhaling of bearing lubricant during heat buildup and the inhaling of coolant as the live center is often flooded at the beginning of a cut.
In constructing a live center it is essential to maintain concentricity between bearing bores and thrust carrying shoulders. Spindles should be round and have a true running thrust carrying flange. Bearings should fit both the body and spindle with a moderate press fit. Class of bearings has little to do with a live center's accuracy if the point is ground after assembly in its own bearings under simulated work conditions. This method in the live center, to be hereinafter described, with a close supervision normally produces a total indicator reading (TIR) of 0.000010 to 0.000050. Automatic preload takeup is also essential to maintaining constant radial and axial precision and provides maximum bearing life.